Stereospecific sp3 C–H oxidation with m-CPBA: A CoIII Schiff base complex as pre-catalyst vs. its CoIIICdII heterometallic derivative

Abstract The mono- and binuclear Schiff base complexes [CoL 3 ]·DMF ( 1 ) and [CoCdL 3 Cl 2 ]·0.5H 2 O ( 2 ) were facilely synthesized using zerovalent cobalt and cadmium chloride (for 2 ) as starting materials and the pre-formed pro-ligand HL (2-methoxy-6-[(methylimino)methyl]phenol, the product of condensation of o -vanillin and methylamine) in air. The compounds were characterized by single crystal X-ray diffraction analysis and spectroscopic methods in solution and in the solid state. Both complexes demonstrate a profound catalytic activity in the stereoselective oxidation of cis- 1,2-dimethylcyclohexane (model substrate) with m -CPBA ( m -chloroperbenzoic acid) under mild conditions in the presence of promoters of various acidity (HNO 3 , TFA and HOAc). The heterometallic binuclear Co III Cd II pre-catalyst ( 2 ) was more active than the mononuclear Co III one ( 1 ), exhibiting higher products yields up to 51% and excellent stereospecificity (up to 99.2% retention of stereoconfiguration). This result could be associated with a synergistic effect of two different metals in 2 . Based on the large obtained kinetic isotope effect and H 2 18 O labeling studies, the overall reaction mechanism was proposed to proceed without the participation of free alkyl radicals. The acidity of the promoter was shown to influence catalytic parameters for both 1 and 2 so that the better parameters are achieved with the acid possessing lower p K a values (a stronger acid). The comparison of the catalytic behaviours of 1 and 2 is discussed in detail considering relevant examples from the literature.